Packaging IC sockets are often used within general electronic equipment. For instance, a socket is used as a sort of connector for the purpose of mounting LSI packages on wiring boards. LSIs mounted on wiring boards using a socket are inspected in the form of wiring modules. If there is something wrong in an LSI, that LSI may then be replaced by another LSI. LSIs in existing modules may be removed out of sockets for replacement with the latest ones.
Packaging IC sockets are required to have the shape and function commensurate with LSI packages, and must also have the specifications corresponding to the pitch and shape of LSI package terminals. There are pin grid arrays (PGAs) whose terminals are of rod shape and ball grid arrays (BGAs) whose terminals are of spherical shape. To engage sockets with these terminals, the shapes of sockets' contacts and how to fit the sockets must be well fit for the respective terminals.
Recent trends in LSI packaging are toward bare chip packaging or flip chip packaging with no LSI package. Associated sockets, too, must be revamped to keep up with such trends. Packaging IC sockets, like other packaging parts such as wiring boards, are now required to have smaller size, higher reliability, higher density (narrower pitch), higher transmission speed, lower noises, etc.
In semiconductor fabrication processes, on the other hand, measuring IC sockets are employed for inspections by burn-in testing. The burn-in testing is one of the screening methods for eliminating initial defects in semiconductor devices, wherein accelerated stress is applied to the devices at temperatures and voltages higher than in their operating conditions to accelerate the occurrence of defects, thereby screening out defective devices within short time spans.
In one typical burn-in testing method, bare chips or packaged devices are located within IC sockets in a high-temperature tank, and power source voltages or input signals providing accelerated stresses are externally applied thereto for a constant time. Removed out of the sockets, whether the devices are accepted or rejected is then determined.
FIG. 1 is a sectional schematic illustrate of one example of the measuring IC socket. A bare chip 1 comprising a bare chip body 11 and bumps 12 is placed on the table of an IC socket 2. The IC socket 2 is made up of a socket body 21, contact probe pins 22, a lid 23, etc. For the pins 22 to come into contact with terminals such as bumps for conduction, for instance, deflection pins or insert pins may be used. Burn-in testing is carried out while the lid 23 is closed. After the completion of the burn-in testing, bare chips judged as acceptable are mounted on board substrates in a multi-chip packaging fashion.
FIG. 2 is a top schematic illustrative of one example of a burn-in socket for fine ball grid array (FBGA) packaging, and FIG. 3 is a sectional schematic of the socket. A socket 3 is built up of contact pins 31 arranged at a constant pitch, a lid 32, springs 33, a body (bottom) 34, etc.
In FIG. 3, pins 31′ extending conductively to a testing machine are also depicted. An FBGA package 35 is placed on a table while it is in contact with the pins 31. The pins 31 are designed in terms of shape, pitch, number, etc. so that they can come into contact with a number of spherical terminals.
IC sockets are now required to have good electrical insulation properties, withstand voltage, mechanical properties, heat resistance, chemical resistance, dimension stability, etc. Some IC sockets are formed of ceramic materials or metals having insulated surfaces. In most cases, however, they are formed of synthetic resins.
In view of the aforesaid electrical insulation properties, hat resistance, etc., IC sockets of synthetic resins, for instance, are formed of thermoplastic resin of high heat resistance such as polyether imide, poly(phenylene sulfide), polysulfone, poly(ether sulfone) and polybutylene terephthalate (JP-A's 07-179758, 08-176441, 08-176442 and 2000-150094).
IC sockets of synthetic resins, because of being required to have a high degree of insulation resistance, have generally high surface resistivity. IC sockets having too high surface resistivity are likely to be deposited on their surface with static electricity. As semiconductor devices are in touch with IC sockets in the inspection process of the semiconductor devices, IC sockets are susceptible to electrification via triboelectrification.
As the packaging density of semiconductor devices becomes high, on the other hand, the number of input/output pins of chips increases with narrowing pitch. When multi-pin PGAs or BGAs, bare chips or the like are mounted on IC sockets, their contact with the surfaces of IC sockets makes damage to these semiconductor devices likely. Adsorbed to the surfaces of IC sockets by static electricity, airborne dust contaminates semiconductor devices.
The surface resistivity of IC sockets may be lowered by forming IC sockets using synthetic resins with antistats added thereto. However, the antistatic agent present on the surface of an IC socket is readily removed by washing or friction, and so it is difficult to achieve the given antistatic effect over an extended period of time. When a large amount of the antistatic agent is added to sustain the antistatic effect, the antistatic agent bleeds on the surface of the IC socket, resulting in the deposition of dust thereon. In addition, the bleeding antistatic agent leads to possible pollution of surrounding environments through elution and vaporization.
The surface resistivity reduction may be achieved by forming IC sockets using synthetic resins with conductive fillers filled therein. However, resin compositions comprising synthetic resins and conductive fillers having extremely low electrical resistivity such as conductive carbon blacks, graphite, carbon fibers, metal fibers, and metal powders suffer from drastic electrical resistivity changes depending on the proportion of the conductive filler incorporated and delicate fluctuations of the state of dispersion of the filler.
Possible reasons for this could be that there is an extraordinary difference in electrical resistivity between conductive fillers and synthetic resins, and that when individual conductive filler particles are dispersed in a synthetic resin while they are joined to one another, conductivity appears drastically because the occurrence of conductivity is dependent on the state of dispersion of a conductive filler.
In addition, the state of dispersion of the conductive filler in the synthetic resin varies from site to site, and so the resulting product tends to take a form wherein a portion having very high surface resistivity and a portion having very low surface resistivity coexist. With such a process using conductive fillers, it is thus still difficult to achieve stable production of IC sockets having their surface resistivity brought down to the desired level while keeping electrical insulating properties.